In general, photographic materials having a silver halide emulsion layer are subject to various outside pressure. For example, negative films for general photography are apt to be bent on rolling in a cartridge or loading into a camera or pulled or rubbed with a carriage part of a camera on feeding. Sheet films such as printing films and direct radiographic films for medical use are often bent on handling with hands. When handled in daylight conveying equipment or high-speed changers, photographic materials are brought into contact with metallic or rubber parts with strong force. All kinds of photographic materials receive great pressure on cutting and fabricating.
Pressure thus applied to a photographic light-sensitive material is transmitted to silver halide grains through gelatin, a binder for silver halide grains, or other high-molecular weight substances as a mediator. It is known that pressure application to silver halide grains causes blackening irrespective of exposure amount or desensitization. For the details, reference can be made, e.g., to K. B. Mather, J. Opt. Soc. Am., Vol. 38, p. 1054 (1948), P. Faelens and P. de Smet, Sco. et Ind. Photo., Vol. 25 No. 5, p. 178 (1954), and P. Faelens, J. Photo. Sci., Vol. 2, p. 105 (1954).
It has therefore been demanded to provide a photographic light-sensitive material whose photographic performance is unaffected by pressure.
On the other hand, high-temperature rapid processing of photographic materials has been rapidly spread, and a time required for various light-sensitive materials to be processed in an automatic developing machine has been greatly reduced. Particularly in ultra-rapid processing, efforts have been made to further raise a drying speed in an automatic developing machine.
Speed-up of drying is generally achieved by adding a sufficient amount of a hardening agent to a light-sensitive material so as to reduce a water content before starting drying in an automatic developing machine. Though successful in increasing a drying speed, this means is attended by many disadvantages. That is, enhanced film hardening results in reduction in sensitivity which leads to retardation of development, reduction in covering power even when tabular grains having a high aspect ratio are used, worsening of color remaining, retardation of fixing of silver halide grains, increase of hypo remaining in a processed light-sensitive material, and the like.
Reduction in water content before starting drying can also be achieved by decreasing hydrophilic substances in a light-sensitive material, i.e., gelatin, synthetic high polymers, and hydrophilic low-molecular weight substances. However, a decrease of these hydrophilic substances means a decrease of a ratio of a binder to silver halide grains, which often causes sensitization or desensitization on scratching or bending during handling before development processing particularly in using tabular grains of high aspect ratio. Hence, without any means to improve pressure resistance, it has been difficult to obtain improved drying properties by decreasing a binder.
On the other hand, there has been a long and constant demand for an emulsion having higher photographic speed. An emulsion of high photographic speed makes it feasible to take a photograph without flashlight even at night and to take a photograph of a fast-moving subject at a high shutter speed. When applied to radiography, it would reduce an X-ray exposure dose to minimize the influence of X-ray on human bodies.
It is well known in the art that hydroxyazaindene compounds have a property to suppress chemical ripening with sulfur-containing compounds and are therefore useful as an emulsion stabilizer. They are added to a photographic emulsion for the purpose of stopping a sulfur sensitization reaction and/or preventing fog during preparation, preservation or developing processing. These compounds are also known to increase a photographic speed. For example, British Patent 1,315,755 discloses a method for carrying out sulfur-gold sensitization of a silver halide emulsion, in which an azaindene compound is added to an emulsion before sulfur sensitization and, either simultaneously or thereafter, a monovalent gold complex compound containing sulfur is added, followed by ripening to obtain a silver halide emulsion having higher intrinsic sensitivity than in conventional methods. Further, JP-A-50-63914 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and German Patent Application (OLS) No. 2,419,798 disclose a method of obtaining increased sensitivity by adding a hydroxytetraazaindene compound to a sulfur-sensitized mono-dispersed emulsion of cubic silver halide grains having a silver bromide content of 80 mol % or more. These references also describe that this method, when applied to silver halide grains other than cubic grains, e.g., octahedral grains and tabular grains which are substantially surrounded by (111) planes, rather results in a reduction in sensitivity or brings about only slight improvement in sensitivity, if any. Further, JP-A-51-77223 teaches that addition of a certain hydroxytetraazaindene compound to a sulfur-sensitized silver halide emulsion whose mean grain size does not exceed 0.5 .mu.m brings about an increase in sensitivity. However, a hydroxytetraazaindene compound has been commonly added as an emulsion stabilizer after chemical ripening irrespective of whether or not the effect of increasing sensitivity may be obtained with or without recognition of that effect. Therefore, the methods disclosed in JP-A-50-63914 and JP-A-51-77223 are not expected as novel techniques for increasing sensitivity.
JP-A-58-126526 proposes a method for preparing a photographic emulsion having a high sensitivity and markedly low fog, in which chemical sensitization of octahedral or tetradecahedral silver halide grains is carried out in the presence of an azaindene compound. Further, JP-A-2-68539 discloses a method for preparing a high sensitivity and low fog emulsion, in which chemical sensitization of tabular grains having an aspect ratio of 3 or more is carried out in the presence of a sensitizing dye and an azaindene compound. Further, it is known that tabular grains are superior to spherical grains for use in X-ray films in view of their higher covering power (optical density per unit silver amount) and higher susceptibility to color sensitization.
On the other hand, there is an unfavorable correlation between photosensitivity and pressure sensitivity. That is, as photosensitivity increases, pressure sensitivity also increases. Moreover, a sensitizing dye promotes the property of silver halide grains to cause fog on application of pressure. In other words, if a large quantity of a sensitizing dye is used for color sensitization in an attempt to increase light absorption and to increase sensitivity, it follows that blackening on pressure application is remarkably emphasized. As a means to be taken against such a disadvantage, it is known to incorporate a plasticizer for polymers or emulsions or to reduce a silver halide/gelatin ratio to thereby prevent applied pressure from reaching silver halide grains.
Known plasticizers include heterocyclic compounds as disclosed in British Patent 738,618, alkyl phthalates as disclosed in British Patent 738,637, alkyl esters as described in British Patent 738,639, polyhydric alcohols as disclosed in U.S. Pat. No. 2,960,404, carboxyalkyl cellulose as disclosed in U.S. Pat. No. 3,121,060, paraffin and carboxylic acid salts as disclosed in JP-A-49-5017, and alkyl acrylates and organic acids as disclosed in JP-B-53-28086 (the term "JP-B" as used herein means an "examined published Japanese patent application").
Since addition of a plasticizer causes a reduction in mechanical strength of an emulsion layer, there is a limit in the used amount of a plasticizer. Further, an increase of gelatin results in retardation of development and reduction in sensitivity. Accordingly, sufficient effects on improving pressure characteristics can hardly be obtained by either of the above-described means.
In general, silver halide grains having a cubic or octahedral crystal form or a potato-like spherical form are less liable to deformation under an outer force because of their shape and have therefore lower pressure sensitivity than tabular grains having a large projected area diameter/thickness ratio. Owing to this advantage, as far as the above-mentioned means for improving pressure characteristics are applied to these grains, some improvements on pressure characteristics could be reached to not a sufficient degree but to a fairly satisfactory level.
Turning now to tabular grains, they have a merit to provide high optical density with a reduced silver amount because of their high covering power per unit area as described in U.S. Pat. Nos. 4,434,226, 4,439,520, and 4,425,425. In addition, they have a large surface area per unit volume and are accordingly capable of adsorbing a larger quantity of a sensitizing dye in spectral sensitization, thus exhibiting higher light capturing ability. Such an advantage can be made best use of by using a sensitizing dye in an amount of 60% or more, preferably 80% or more, and more preferably 100% or more, of a saturation adsorption. As previously stated, however, pressure sensitivity increases with the amount of a sensitizer. Besides, the shape of tabular grains makes them liable to deformation on application of an outer force. For these reasons, the above-described means cannot achieve a satisfactory improvement in pressure characteristics with tabular grains.
JP-A-64-72141 suggests to reduce pressure blackening by adding a polyhydroxybenzene compound to tabular grains. Since this method is accompanied by a reduction in sensitivity, sufficient improvement cannot be reached when high sensitivity is required.
Hence, it is essential to develop a technique for improving pressure resistance of a light-sensitive material which is required to have high sensitivity and suitability for ultra-rapid processing.
In particular, in the field of radiographic materials for medical use including X-ray films, rapid processing has great advantage that quick completion of development processing would permit of a timely medical treatment. A number of studies have thus been made on rapid development processing of X-ray films.
X-ray films have conventionally been processed in a dry-to-dry time (from the beginning of development processing to the end of drying processing) of about 90 seconds. With the developments of rapid processing, the dry-to-dry time has recently been reduced to about 45 seconds. In order to sufficiently agree with the latest medical advancement, there has been a need for further speed-up of processing. For example, ultra-rapid development processing requiring a dry-to-dry time of not more than 30 seconds is desired.
If conventional light-sensitive materials are subjected to ultra-rapid processing in a dry-to-dry time of 30 seconds, there have arisen various problems: for example, a processed light-sensitive material cannot be sufficiently dried; where a binder is reduced to improve drying properties, unevenness of development results; and photographic properties are largely varied on pressure application. In order to overcome development unevenness and to improve pressure resistance while improving drying properties by reducing a binder, there has been no means but to reduce photographic sensitivity.
It has been demanded to develop a light-sensitive material which exhibits sufficient performance properties even when processed in a dry-to-dry time of not more than 30 seconds.